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AO map with perspective distortion

Zyth
polycounter lvl 12
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Zyth polycounter lvl 12
Hi all.

I'm having an issue creating an AO map using Mental Ray in 3ds Max. I wanted to do a quick and dirty AO map just to see how it would look. I was pretty pleased until I noticed that it appeared to have perspective distortion in it (i.e. it's not orthographic).

I've attached an image with the areas of greatest distortion circled. I've looked through the Render settings and the Render to Texture settings but couldn't find anything recognizable that might fix the issue. However, since I'm new to this it could very well be something obvious.

I'm at a loss. Any help would be appreciated.
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  • passerby
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    passerby polycounter lvl 12
    whats your bakeing cage look like?
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  • ZacD
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    ZacD ngon master
    It's an issue with the cage and soft edges, there's a few solutions, you could add an edge that goes through anything that has a lot of distortion, you could flatten the detail on the high poly so their less depth, you could make the cage be smaller/match the low poly better. There use to be a free video on 3d motive that explained this pretty well, but they switched it with a less informative video.
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  • Zyth
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    Zyth polycounter lvl 12
    Hey thanks for the info so far. I'm not sure what the best way to show you what the cage looks like so I put together a few views. It's pretty much a straight forward cylinder minus the top. Some of the cage is pulled out a bit in sections to fit the protruding geometry.
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  • ZacD
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    ZacD ngon master
    A more detailed explanation is the surface normals of the cage dictates the angle in which the high poly is captured. If the lowpoly and cage are completely flat, the highpoly will be captured as completely flat. If there is a flat part on the low poly that leads into a sloped area, you'll see on the smoothed normals that the vertex normals aren't flat in that area, so if you have any highpoly details on that flat area with angled normals, the projection will be skewed.

    Like if you look at this low poly

    NormalMap?action=AttachFile&do=get&target=BenMathis_SmoothingGroups_Excerpt.gif

    If there was tall highpoly detail on the angled flat areas, they'd project differently with and without the hard edge.



    EDIT: You should split the top and bottom from the rest of the cylinder, that should fix any issues.
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  • joeriv
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    joeriv polycounter lvl 7
    wouldn't the correct way be to just add control edges to the lowpoly?

    I never heard of actually splitting the geometry, but it seems that it would be better to just add control edges, for when later on this happens on more complex things where you can exactly split it.

    Or am I missing something?



    And a very quick thing for the threadstarter that may make clear what control edges do in this case.

    Left one is what you have now, and how the projection sees your highpoly model, due to the corners, and the way it sort of averages it's looking slanted at your model where it should be straight.
    on the right 2 control edges were added (green), wich makes sure that the projection is straight where it should be.

    thingyf.jpg
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  • ZacD
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    ZacD ngon master
    You can split the cage and leave the lowpoly together, the uv's are going to be spit at the top and bottom, there's no reason to add geometry when you can fix the issue without using more tris.
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  • EarthQuake
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    EarthQuake mod
    ZacD wrote: »
    You can split the cage and leave the lowpoly together, the uv's are going to be spit at the top and bottom, there's no reason to add geometry when you can fix the issue without using more tris.

    wut

    Splitting the cage = seams, so you're just trading one problem for another.

    also: http://www.polycount.com/forum/showthread.php?t=81154
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  • Zyth
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    Zyth polycounter lvl 12
    Yup, the control edges did the trick. Thanks for all the help and thanks for the link EarthQuake.
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